MAINT: Adds a tests/utils/ directory and renames test_utils.py
 to utils/test_factory.py
MAINT: Minor tweaks to docstring.
MAINT: In _create_generator only overwrite perf_tracker when it's None.
MAINT: Use filter builtin instead of if-statement and continue.
DOC: Slightly better comment on Frequency._ffill.
DOC: Clarificatory comments in HistoryContainer.
MAINT: Remove unused function in test_rolling_panel.py.
add account object to context